Just exactly how your GPS device functions to accurately plot your position is the subject of a story on PC World.
Differential GPS systems focus on high precision. One of the best known is NASA’s Global Differential GPS System (GDGPS), developed originally by the Jet Propulsion Laboratory (JPL), run by California Institute of Technology under a NASA contract. NASA claims it’s the biggest such network, originally built to support its own terrestrial, airborne and space operations, but now available to government and commercial customers.
Some are mainly research oriented, like that of NASA’s JPL.
Another example, using a different approach, is the StarFire Network from Navcom Technology (now a unit of John Deere Company), which specializes in very high-precision (10 centimeters) GPS applications such as land and aerial surveying, precision agriculture, and machine control. Navcom says it’s the first satellite-based augmentation system, providing higher accuracy and the ability for users to roam anywhere without being “tethered” to a nearby ground station for augmentation data.